The present invention relates to sports equipment, and more particularly to damping, controlling vibrations and affecting stiffness of sports equipment, such as a racquet, ski, or the like. In general, a great many sports employ implements which are subject to either isolated extremely strong impacts, or to large but dynamically varying forces exerted over longer intervals of time or over a large portion of their body. Thus, for example, implements such as baseball bats, playing racquets, sticks and mallets are each subject very high intensity impact applied to a fixed or variable point of their playing surface and propagating along an elongated handle that is held by the player. With such implements, while the speed, performance or handling of the striking implement itself maybe relatively unaffected by the impact, the resultant vibration may strongly jar the person holding it. Other sporting equipment, such as sleds, bicycles or skis, may be subjected to extreme impact as well as to diffuse stresses applied over a protracted area and a continuous period of time, and may evolve complex mechanical responses thereto. These responses may excite vibrations or may alter the shape of runners, frame, or chassis structures, or other air- or ground-contacting surfaces. In this case, the vibrations or deformations have a direct impact both on the degree of control which the driver or skier may exert over his path of movement, and on the net speed or efficiency of motion achievable therewith.
Taking by way of example the instance of downhill or slalom skis, basic mechanical considerations have long dictated that this equipment be formed of flexible yet highly stiff material having a slight curvature in the longitudinal and preferably also in the traverse directions. Such long, stiff plate-like members are inherently subject to a high degree of ringing and structural vibration, whether they be constructed of metal, wood, fibers, epoxy or some composite or combination thereof. In general, the location of the skier's weight centrally over the middle of the ski provides a generally fixed region of contact with the ground so that very slight changes in the skier's posture and weight-bearing attitude are effective to bring the various edges and running surfaces of the ski into optimal skiing positions with respect to the underlying terrain. This allows control of steering and travel speed, provided that the underlying snow or ice has sufficient amount of yield and the travel velocity remains sufficiently low. However, the extent of flutter and vibration arising at higher speeds and on irregular, bumpy, icy surfaces can seriously degrade performance. In particular, mechanical vibration leads to an increase in the apparent frictional forces or net drag exerted against the ski by the underlying surface, or may even lead to a loss of control when blade-like edges are displaced so much that they fail to contact the ground. This problem particularly arises with modem skis, and analogous problems arise with tennis racquets and the like made with metals and synthetic materials that may exhibit much higher stiffness and elasticity than wood.
In general, to applicant's knowledge, the only practical approach so far developed for preventing vibration from arising has been to incorporate in a sports article such as a ski, an inelastic material which adds damping to the overall structure or to provide a flexible block device external to the main body thereof. Because of the trade-offs in weight, strength, stiffness and flexibility that are inherent in the approach of adding inelastic elements onto a ski, it is highly desirable to develop other, and improved, methods and structures for vibration control. In particular, it would be desirable to develop a vibration control of light weight, or one that also contributes to structural strength and stiffness so it imposes little or no weight penalty. Other features which would be beneficial include a vibration control structure having broad bandwidth, small volume, ruggedness, and adaptability.
The limitations of the vibrational response of sports implements and equipment other than skis or sleds are somewhat analogous, and their interactions with the environment or effect on the player may be understood, mutatis mutandi. It would be desirable to provide a general solution to the vibrational problem of a sports article. Accordingly, there is a great need for a sports damper.
It should be noted that in the field of advanced structural mechanics, there has been a fair amount of research and experimentation on the possibility of controlling thin structural members, such as airfoils, trusses of certain shapes, and thin skins made of advanced composite or metal material, by actuation of piezoelectric sheets embedded in or attached to these structures. However, such studies are generally undertaken with a view toward modeling an effect achievable with the piezo actuators when they are attached to simplified models of mechanical structures and to specialized driving and monitoring equipment in a laboratory.
In such cases, it is generally necessary to assure that the percentage of strain energy partitioned into the piezo elements from the structural model is relatively great; also in these circumstances, large actuation signals may be necessary to drive the piezo elements sufficiently to achieve the desired control. Furthermore, since the most effective active strain elements are generally available as brittle, ceramic sheet material, much of this research has required that the actuators be specially assembled and bonded into the test structures, and be protected against extreme impacts or deformations. Other, less brittle forms of piezo-actuated material are available in the form of polymeric sheet material, such as PVDF. However, this latter material, while not brittle or prone to cracking is capable of producing only relatively low mechanical actuation forces. Thus, while PVDF is easily applied to surfaces and may be quite useful for strain sensors, its potential for active control of a physical structure is limited. Furthermore, even for piezoceramic actuator materials, the net amount of useful strain is limited by the form of attachment, and displacement introduced in the actuator material is small.
All of the foregoing considerations would seem to preclude any effective application of piezo elements to enhance the performance of a sports implement.
Nonetheless, a number of sports implements remain subject to performance problems as they undergo displacement or vibration, and are strained during normal use. While modern materials have achieved lightness, stiffness and strength, these very properties may exacerbate vibrational problems. It would therefore be desirable to provide a general construction which reduces or compensates for undesirable performance states, or prevents their occurrence in actual use of a sports implement.